Political online game and economic simulation

How can I decrease my national debt. And what tax rate is ideal for my state?

Well, you've made things much more difficult by enforcing two expensive reforms - universal healthcare and basic income. You really need to cut back on spending in these areas as you just can't afford it.
There'll have to be some tax increases too. Looking at your current rates, I'd look to raise excise taxes as this will get you some more income and does less damage to the economy than raising acquisition taxes.

A tax reform might give your economy a bit of a boost and help to increase your tax take and you could cut interest rates to make your debt slightly less expensive to service. Money Reform II would wipe out your debt, but you need to get more of a balance between income and expenditure before you try this.

You might want to try freigeld after doing the Money reform II, because your nominal interest will skyrocket. Also this is proof that universal health care in this game doesn not really increase the health index, nor reduce enough the mortality rate, and you have more than a trillion spending in health care.

Simple, raise excise and aquisition taxes to 21-22% each, and it will hurt your economy but it is time for tough choices and your economy wont suffer that much from the increase. Next you decrease both health and Basic income spending as much as you can, try to get health down to at least $400bil. and try to get Basic Income below $300bil. and increase welfare and pension spending to the cost of living level. I know this is hard for your ideals but your $22tril. in debt, thats more than triple what you started with and you can afford anything your doing right now, you have to get rid of a $4tril. deficit and you need to get your finances in check before your do anything else. Because at this point it cut, cut, cut!

I consider money reform 2 to be a bit of a trap. The game creators were correct to put in all kinds of choices but many of them are false choices and traps. The key is to make your choices wisely and avoid the pitfalls. They were wise to make many choices available and it is up to us to sort through them.
If you enact fiscal discipline you can balance your budget and then the write-off would be a matter of impatience as you could wipe out your debt over a multi-year span.
If your budget is not balanced then you would get the interest rate rise, a hit to your international reputation and then you would sink back into debt and will have exhausted your ammunition as you cannot enact MR2 twice.

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I can't help but wonder if maybe it COULD increase it but it would require a constant raising of the health budget?- possibly to budget-busting levels? Also if universal care is enacted would the game assume that costs were relieved from the private sector and give an economic boost to the private sector since the health care premiums would be transferred from the private to the public sector? I have ideas, theories and evidence on how that would work in real life but it still is murky on how the game treats these things.

I went for Privatized Healthcare myself actually. PE Health takes a nosedive and a spending increase on it will help to counter the hit you take to Health while still being well below what you used to spend on it.

Labour time: 6.70 h/Day
Free time: 7.43 h/Day

Your population seriously needs some incentive to get off their lazy asses. Also, in my humble opinion your insanely high Basic Income and PE Health is killing your state, i don't think you can pull out of this one at -4 tril balance (-1,7 tril being debt interest) without some serious debt relief though.